In terrestrial transmission systems, large capacity optical transmission via Wavelength Division Multiplexing (WDM) transmission in C-band is investigated carefully. In such systems, optical transmission lines are constructed using optical fibers on the ground. The C-band refers to wavelengths between 1530 and 1565 nm. As for optical transmission lines in terrestrial transmissions, single-mode optical fibers, which have a positive chromatic dispersion of approximately 4˜16 ps/(nm·km) at 1550 nm, are primarily used. Above all, because of its high manufacturing capability, low loss, and low non-linear characteristics, standard single-mode optical fibers (SMF) are currently the most popular optical fiber used in optical transmissions. SMF has chromatic dispersion of 16 ps/(nm·km) at 1550 nm. Dispersion was a significant issue in the past; however, thanks to the development of module-type dispersion compensating fibers, the issue was resolved (refer “L. Gruner-Nielsen et al., “Dispersion-Compensating Fibers” J. Lightwav. Tech., Vol. 23, No. 11, pp. 3566-3579 (2005)”).
Also, SMF has positive chromatic dispersion around 1310 nm, and cutoff wavelength of shorter than 1300 nm. These make the SMF to be used in transmitting a broadband WDM optical transmission in single-mode. Recently, because of SMF with a lower absorption peak (caused by OH-ions in the optical fiber) at 1380 nm, and with the development of amplifying device technology, SMF can now use the C-band, the L-band (wavelengths of 1565˜1620 nm), and the S-band (wavelengths of 1460˜1530 nm) for optical transmission.
To increase optical transmission distance and transmission capacity by using SMF, it is desirable to further reduce nonlinearity of the optical fiber, which causes deterioration of optical signals. An effective core area of conventional SMF is approximately 80 μm2. However, if the effective core area is further increased, then the energy density of light within the core decreases. It makes possible to create an optical fiber with even lower nonlinearity. For example, as optical fibers to be used in submarine transmission, optical fibers with their effective core areas of approximately 118 μm2 are disclosed in “K. Nagayama et al., “Ultra-low-loss (0.1484 dB/km) pure silica core fibre and extension of transmission distance” Electron. Lett. Vol. 35, Issue 20 pp. 1168-1168 (2002)”. Also, optical fibers with even larger effective core areas are disclosed in “K. Aikawa et al., “Single-mode Optical Fiber with Effective Core Area Larger than 160 μm2” ECOC '99, I, 302-303 (1999)” and “M. Tsukitani et al., “Ultra Low Nonlinearity Pure-Silica-Core Fiber with an Effective Area of 211 μm2 and Transmission Loss of 0.159 dB/km” ECOC '02, 3.2.2 (2002)”.
However, the effective core areas of the above conventional optical fibers are increased by making their cutoff wavelength longer and/or increasing their bending loss. They create issues such as narrow useful wavelength band for single-mode transmission and increase in macrobending loss.